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Why is Bowl Now Out of Round?

Discussion in 'Getting Started' started by Dan Bevilacqua, Jun 27, 2018.

  1. Dan Bevilacqua

    Dan Bevilacqua

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    Turning a piece of 8/4 kiln dried lumber yard purchased walnut that I cut into a circle about 7" in diameter. I turned the outside on a face plate then mounted the recess in a chuck. It was only little off as far as runout was concerned after I mounted it in the chuck. I then turned the outside again and got it to where there was only about .001" runout (used my tool rest against the outside of the side of the bowl similar to photos). I assume this is normal.

    Now the issue. I turned the inside of the bowl, and now there is about .050" runout (almost 1/16") on the outside side of the bowl (again, used my tool rest against the outside side of the bowl similar to photos).

    You can see in the rim of the bowl (looking to the inside) that the rim at the edges that run with the grain significantly thicker than the edges that run with the face grain. Any help would be appreciated.

    By the way, I still need to sand.

    100_0691.JPG 100_0693.JPG
     
  2. Mark Wollschlager

    Mark Wollschlager

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    I can offer a couple of thoughts.
    If the faceplate was not registered against the flat of the spindle your recess would not be completely in the center and give you wobble.
    The contact surface between the wood and faceplate might have been a bit rough, unless the blank was planed on both sides.
    Likewise, if the chuck spindle adapter was not flat against the back of the spindle the piece might be off.
    If the recess is not cut very cleanly with a flat bottom, wobble can be introduced even if you use the tailstock to press it in the chuck.
    If you have a rotating headstock, I would check the alignment with the tailstock.
    Crud under the headstock can cause it to be slightly off even if tightened down.
    The wood can move once stresses are removed. This can happen with kiln dryed as well as green wood.

    So checking alignment, making sure all contact surfaces and the spindle threads are clean before screwing chucks and faceplates on.
    Clean tenons and recesses.
    Just some places to start.
     
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  3. Clifton C

    Clifton C

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    Dry in this case isn't the issue. Someone here does some really nice incised lines but it might be a secret:D...
    When I have a piece like this that wants to move, I start the way you do (cept I'm a tenon guy on bowls) After I flip it around, I start with the rim and only work the first 1/2 to 3/4 in into the bowl, leaving the mass of wood in the center, After I've refined the rim, I work a little farther down, working in steps. By now, you cannot go back to the rim, it has already started to move. When I reach about 2/3rds down, I start taking out the center, again, you can never go back to the rim...it has moved. If you just excavate the bowl without leaving some mass in the center, the arrows show the movement of the bowl.
    Stretched bowl.jpg
     
    Last edited: Jun 27, 2018
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  4. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Kiln dried lumber is generally around 20% moisture content while EMC (equilibrium moisture content) is usually in the 8 - 12% moisture content depending on the species and your local climate. Even wood that has been dry for years will still move when material is removed during the turning process. There are multiple reasons why this can happen, but they all boil down to internal stresses that were in equilibrium are suddenly no longer in equilibrium ... the end result is that the wood moves until internal stresses are back in equilibrium. The usual solution is to rough turn the bowl leaving the walls extra thick to allow for warping and then a few months later do the final turning.

    A couple other thoughts.
    1. A bowl with a flat bottom and vertical sides will be more problematic with respect to drying problems than a bowl that has a round cross section. The reason is that the round cross section bowl doesn't have the problems associated with an abrupt change in grain orientation that a bowl with a flat bottom and vertical sides does.
    2. Wood never completely stops moving. Although it's fairly stable after reaching EMC it still moves with seasonal changes. Wood generally has good dimensional stability with moisture changes along the direction of the grain, but the cross grain movement may be two or three (or more depending on species) times greater.
     
  5. Dan Bevilacqua

    Dan Bevilacqua

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    Thanks for the responses.

    Mark, I think your mention of wood movement is what is going on.

    Clifton, your thoughts on leaving the center of the inside intact until toward the end is possibly what may be going on because I didn't do that. I removed the interior of the bowl in one movement from the rim to the center. I' ll try your method next time.

    Bill, your theory is well stated.

    I guess the bottom line is that the wood moved. I was surprised with kiln dried wood because I thought that only wet wood moved so much (actually, I've had occasions of less movement with somewhat wet wood). I guess I caused the significant movement due to a flat bottom and relatively steep sides as well as not leaving some center support in the interior until almost do with the interior.

    Any other tips so that I can avoid this in the future would be appreciated. Thanks, again.
     
  6. hockenbery

    hockenbery AAW Advisor Staff Member

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    What tools are you using? Did you sand it? If so what grits?

    With 7” diameter kiln dry walnut 2”thick - I would expect small movement due drying like 1/64”
    Smaller bowls just don’t have the movement issue we see inlarger ones.
    Check the recess walls
    If your recess has any torn grain from a scraper or parting tool it will likely not center the bowl well.

    If you have sanded the bowl, I would suggest the sanding has brought it out of round
    Sanding with 120 will take more wood off the softer long grain than the Harder endgrain.
    Walnut is fairly soft.
    Sanding on the outside and inside with the lathe running will make the long grain sides thinner.

    Also Scraping the walls can also remove more wood from the long grain than the endgrain.
     
    Last edited: Jun 28, 2018
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  7. Dan Bevilacqua

    Dan Bevilacqua

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    Thanks, hockenbery. To answer your questions, I have not yet sanded; except, I did finish the recess with sanding and with walnut oil while it was spinning on the face plate, before mounting to the chuck.

    I used mostly scrapers to do the recess. I think the recess came out pretty smooth with a dovetail for my Nova G3 chuck. I think the bowl was centered because there was only around .001" runout on the outside side (you can see the tool rest position in the photos which shows how I checked the runout) of the bowl until I removed the material from the inside of the bowl, giving me about .050" runout.

    I did use a scraper to clean up the side walls and bottom of the inside of the bowl.
     
  8. odie

    odie

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    Dan.....If you had sanded, then Al is correct about how sanding does remove more wood from the long grain vs end grain.

    Absent of that, then Bill's explanation applies. I don't recall of a bowl ever not warping to some degree.....some more than others, but always some amount of warp.

    This is strictly a theory of mine, but I believe internal stresses are also subject to moisture loss due to heat......so, if that is true, then light delicate cuts are more likely to produce less heat than heavy aggressive cuts. RPM can have a cooling effect, while bevel rubbing will do the opposite, and create more heat from friction than it would at a slower RPM. I believe that moisture loss (due to heat) is more pronounced at the surface.....or, close to the source of the heat. Afterwards, the interior moisture will self-stabilize, and move toward where the moisture loss has occurred. (This is why surface checking can sometimes occur while roughing a wet bowl, but stops rather quickly......I think! o_O)

    After sanding, then both warping AND sanding will contribute to an out of round condition.

    -----odie-----
     
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  9. Lamar Wright

    Lamar Wright

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    Hi Odie... never thought about this theory but it does make since to me because moisture will move to the driest area of the bowl and could cause warp-age. How would relative humidity play a part in this?
     
  10. john lucas

    john lucas AAW Forum Expert

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    I have found that wood even small wood moves when you remove wood. I do boxes and quite often after I hollow the interior the wood will have moved slightly. Sometimes I will hollow a box and lid and then put them both up to stabilize for a few days before finishing the hollowing and threading the boxes. This eliminates that subtle movement.
     
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  11. odie

    odie

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    Hi Lamar........The relative humidity will have it's effect on how any bowl will stabilize.....and, this will vary, depending on your location, among other things. If a bowl is already roughed and stabilized to your location, then the act of turning it to a final shape might draw in moisture from the atmosphere. (I think this was what you were getting at, and is a good point you are making.) I'm not sure how much an effect this will be, but certainly all pieces of this puzzle are worth adding to the equation. :D

    -----odie-----
     
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  12. Dan Bevilacqua

    Dan Bevilacqua

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    Great responses. Thanks. I guess I'll go sand it and finish it a little later.
     
  13. Dan Bevilacqua

    Dan Bevilacqua

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    I favor making boxes over bowls. I have made a couple of boxes lately (about 3.5" in diameter. I tried using a lid of a previous one that would not fit due to wood movement of the base. The lid was made of maple that I got from a downed tree. I let the wood sit in a brown paper bag for about a year (low humidity around here). appears to be little movement in that piece.

    Anyway, I turned a base out of ash that was a little wet, and the lid fit very nicely before I took the piece off the chuck. The base opening shrunk so much that the diameter of the lid is now as large large as the diameter of the top of the base (base shrank about 1/8" in diameter). I somewhat expected that with the somewhat wet wood.

    So, I made another base for the lid out of box elder maple that was wet. Again, the lid fit very nicely before I removed it from the chuck. Now the fit is about the same as the ash one due to the base shrinking around the same amount as the ash base. Again, I expected that with the wet wood.

    I have made several smaller boxes with kiln dried wood, maple and walnut to be specific, and have had no movement. This is why I was not expecting the movement exhibited by the kiln dried walnut bowl that is that is photographed in the original post.
     
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  14. robo hippy

    robo hippy

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    Wood always moves... When ever I make boxes, no matter how dry the wood is supposed to be, I rough turn them first and then let them sit for a while. When ever you remove wood, what is left will 'adjust' to having the mass removed. As for bowls, that bowl could have moved that much just due to weather changes as in hot dry day to rainy day... Getting a reversed bowl to mount with no run out is next to impossible. I prefer a shear scrape, or negative rake scraper to finish the inside of a recess because it doesn't get the tiny bit of bounce that happens with a bevel rubbing cut.

    Sanding should be done at slower speeds, 500 rpm or less. It is all about traction so the abrasives can cut. If your hand is getting hot from sanding, so is the wood, and you are spinning too fast and/or using too much pressure.

    robo hippy
     
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  15. hockenbery

    hockenbery AAW Advisor Staff Member

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    Wood will move to reach EMC -Equilibrium Moisture Content
    Kiln dried wood is usually about 8% when coming out of the kiln
    Wood sitting in the retail rack will over time get an EMC that corresponds to the RH and temperature of the retail rack.
    Flat lumber folks are advised to let their stock sit in the shop for a week before joinery is cut.
    In a 50% RH environment the wood will stay real close to 8% EMC. See chart below

    Drying movement
    This wood movement is a slow process with dry wood taking many hours. With dead wet wood it can be fairly quick with thin walled bowls. Normally the time between shaping the outside and hollowing a bowl is too short for movement to occur in dry wood due to EMC change. With dead wet wood working the wall to thickness in 1-2” increments from the rim will yield an even wall before the wood moves from to moisture loss.
    The finished turned wet bowl when dry will have no shrinkage on the endgrain wall thickness as wood does not shrink in the vertical growth direction. The wall with the long grain will shrink mostly in the radial direction( bark to tree center) this is typically 5%. So a wall turned 1/4” thick will be 1/4” thick on the endgrain and a Fat 15/64 on the long grain wall.

    Tension wood
    A common type of wood movement is from Tension. Crotches, limbs, wood near the roots have internal tension to support the tree structures. Hollowing the bowl releases the tension and the walls can spring inward or outward.
    This movement happens as soon as the connecting wood is removed. When I hollow a natural edge bowl I reshape the outside after I hollow some to release the tension. This lets the wood move some before I return the outside. Kiln drying can sometimes create tension. Case hardening is a prime example.

    Look up the Temp and RH in the chart below. My shop the RH is around 70% in the summer and 85 degrees
    My drying room has an RH of 50% and 78 degree temperature

    73B6E34B-43DD-4F6A-80CD-91C3F35CF026.jpeg
     
    Last edited: Jun 28, 2018
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  16. Bill Boehme

    Bill Boehme Administrator Staff Member

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    To clarify a point, it's not relative humidity, but absolute humidity that affects the amount of moisture change in wood. And, this is a very slow change that occurs over months. Just as it takes months for wood to dry, the same applies for it to absorb moisture.
     
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  17. Bill Boehme

    Bill Boehme Administrator Staff Member

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    This is known as "reaction" wood. One side of a piece of wood is in tension while the other side is in compression, but one can't exist without the other no matter how small the piece of wood. Reaction wood occurs in leaning trees as well as limbs. Everything is in equilibrium until some wood is removed and upsets the balance between compression and tension. This forces the wood to either warp or even split if the reaction forces are especially high.
     
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  18. hockenbery

    hockenbery AAW Advisor Staff Member

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    I’m not sure these have universal accepted definitions.
    There may well be contradicting definitions.
    If it stops raining i’ll Go out to the shop to check Hoadly.
    What I find in limited research follows the web page below.

    These definitions are contrary to the logical.
    compression wood and tension wood are not opposite structures in the same log.

    References I find suggest there is either tension wood holding up a structure or compression wood pushing up a structure but not both in the same log

    Mostly hardwood use tensionwood and softwoods use compression.

    Stopped raining
    Hoadly supports: Tension wood in hardwoods. Compression wood in conifers

    9AD41F0F-72FC-484A-84A0-7DF6DAB9A26C.jpeg
     
    Last edited: Jun 28, 2018
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  19. john lucas

    john lucas AAW Forum Expert

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    It is my understanding that you have both tension and compression wood in limbs just like Bill described. Don't remember where i read it but it was described as the compression wood is in the bottom of the limb and tension wood on the top side. Guess I'll have to do some more reading. I have Hoadley's book I'll pull it out and see what I can find.
    Hoadley talks about reaction wood but only about a page and half and the photos take up most of the space.
    OK I looked it up. Compression wood is primarily found in Softwoods and is on the underside of limbs or the downhill side or leaning trees. Usually the pith will be off center and the compression wood is on the bottom of the limb or the side where the pith as closest to the bark. Compression wood is weaker than regular wood. Compression wood is on the underside of the limb.
    Tension wood is primarily seen in Hardwoods and can go all the way around the pith. Tension wood is stronger but tends to cut leaving a fuzzy surface. Tension wood is on the upper side of the limb because gravity causes the upper side to be in tension.
    Nowhere does he say that one log can have both but I was sure I read that somewhere.
     
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  20. hockenbery

    hockenbery AAW Advisor Staff Member

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    Until a couple of hours ago I used to think like you guys
    Tension wood on one side Compression wood on the other side.
    Not so From every thing I could find.

    You get one or the other but not both in the same tree.
    hardwoods grow tension wood.
    Conifers grow compression wood.
     
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  21. robo hippy

    robo hippy

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    Hmm, I always thought compression wood was the rippled under where a big branch comes into the tree... Had a madrone piece years ago that was from the buttress part of the base of the tree where it flared out. I turned it as square to the grain as possible, and as it dried, it looked like one side was too close to the heater and it melted/sagged over to the one side...

    robo hippy
     
  22. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    While ago I pointed out to someone on facebook that looked like he had used to much pressure and or speed while sanding, looked like crackled glass... He reply "Why? that was done on purpose to embellish the bowl!! LOL . When I find sanding marks now I tell people it was a design choice...
     
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  23. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    What a great chart, I have to say I have never seen it... Bone dry Koa in Upcountry Maui, we are within walking distance of the rainforest, is 14.6. Down by the beach, changes to 7.8. I never worry much about humidity unless I'm sending the piece to the mainland, specially to a dry place...
     
  24. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    One of our club members is also a carpenter that built houses with an old style of Japanese bulding techniques, no nails, all joint. He is a master at reading the wood, he mills his own. I wanted to mill a big Eucalyptus Citridora, called him for advice and I was so confused within minutes, slabbing is not as easy as it sounds, that is if you dont want your slabs to warp and crack . I decided to cut it for turning stock, much easier...
     

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